1. Field of the Invention
The present invention relates to novel thiazine derivatives which form a color on intimate contact with electron accepting materials and to a process for preparing the same.
2. Description of the Prior Art
Hitherto, 4H-3,1-benzothiazine derivatives represented by the following formula (III) are known as thiazine derivatives, which have been synthesized for use as sedatives (Chemical Abstracts, 70, 87829u): ##STR3## wherein R represents hydrogen, methyl, ethyl, isopropyl, .beta.-diethylaminoethyl, benzyl or benzoyl, R.sub.1 represents hydrogen or chlorine, and R.sub.2 and R.sub.3 each represents methyl, ethyl, propyl or phenyl.
The 4H-3,1-benzothiazine derivatives represented by the formula (III) do not form a color when they contact an electron accepting material, because R.sub.2 or R.sub.3 in the formula is not an electron donating aryl group such as a p-dimethylaminophenyl group. The compounds represented by the formula (III) are synthesized according to the following reaction schematic (1) or (2). ##STR4##
This process is carried out by reacting an o-amino-.alpha.,.alpha.-disubstituted benzyl alcohol prepared by a Grignard reaction with a thioisocyanate and then ring-closing the synthesized o-thioureido-.alpha.,.alpha.-disubstituted benzyl alcohol thus obtained by means of hydrogen bromide or hydrogen chloride to yield 4H-3,1-benzothiazine derivatives. The first problem of this process is to utilize the Grignard reaction. Moisture severely affects the Grignard reaction and thus ethers should be used which give rise to the danger of fire. Therefore, the Grignard reaction is very disadvantageous economically, because special production equipment is necessary in industrial operation.
The second problem is that the corresponding o-amino-.alpha.,.alpha.-disubstituted benzyl alcohol is very unstable and easily forms anhydro-base according to the following reaction schematic (3) where R.sub.2 and R.sub.3 in the reaction schematic (1) or (2) is each a p-dimethylaminophenyl group and R.sub.1 is, for example, a dimethylamino group and, consequently, a thioureido group is difficult to introduce. ##STR5## where Me represents a CH.sub.3 group.